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Traffic Alert/Collision Avoidance System

INTRODUCTION

In 1956, the Civil Aeronautics Administration Technical Development Center reported
that results of tests that had been conducted over the last four years indicate that
only general use of proximity warning devices would substantially reduce the steadily
increasing threat of mid-air collisions.1 The general interest in such
devices was initially spawned by the ever-increasing growth in air traffic. However, the
catalyst for more in-depth research was an accident that occurred as a result of a
collision between two airliners over the Grand Canyon on June 30, 1956. In 1978 a light
aircraft collided with an airliner over San Diego. By this time US pilots began to warm up
to the idea of a collision avoidance system. Ultimately, the final impetus that led to
congressional legislation mandating Traffic Alert/Collision Avoidance System (TCAS) was
the August 31, 1986 midair collision involving and Aeromexico DC-9 and a private airplane
in U.S. airspace over Cerritos, California near Los Angeles International Airport.2
Throughout this period, many versions of midair collision avoidance devices were proposed.
This discussion will explore the evolution of TCAS as we know it today. The specific
characteristics and differences between the systems will also be examined as well as pros
and cons. Additionally, the future of TCAS will be discussed.

EARLY AIRBORNE COLLISION AVOIDANCE SYSTEMS

As mentioned previously, TCAS or other similar devices have been in various stages of
research and development since the early to mid 50s. Research findings during this
time identified that the greatest danger of a collision lies in one aircraft overtaking
another. The research also found that a warning to a pilot that potential collision danger
exists is not sufficient information for prevention of a collision, and that relative
bearing of an existing collision threat must be known to the pilot to give him enough time
to see the other aircraft and execute an avoidance maneuver. Finally, it was discovered
that most collisions occurred in terminal areas. The critical element in approaching a
solution to the midair collision problem was that of time-distance because of the
potential rapid closure rates of jet aircraft converging nose to nose. Tests showed that
when pilots initiate a sudden climb in a jet aircraft traveling at 400 knots, the aircraft
would travel approximately one mile before the it would respond and start to climb.3
Early warning was critical to reducing midair collisions.
With these findings in mind, scientist began to explore the possibilities of developing a
new piece of equipment and installing it in aircraft to protect against midair collisions.
The research became known as Airborne Collision Avoidance System research or ACAS. One of
the earliest collision avoidance systems that was proposed, developed in the 1950s, was a
three range device for high-speed jet aircraft. This was an adjustable device that would
lessen the false alerts in congested areas. The shortest range was used in a congested
terminal environment. The medium range was used for lower altitude flights with the long
range being used while cruising in the flight levels. The adjustments in the system were
made by changing the maximum range and altitude before a conflict alert signal was
received. The earliest systems were based on equipment that attempted to calculate
miss-distance, or the distance at which point the system would recommend an
evasive maneuver. Obviously, if the miss-distance was at a minimum, an evasive maneuver
was suggested. For the system to operate accurately, it required that relative bearing
angle and closure rate between aircraft be calculated. This presented problems in
turbulent conditions and the size of the equipment required was considered excessive. It
never found a market.4
Another type system, developed by Bendix Radio in the late 50s and early 60s,
took a different approach and used time to determine how long before participating
aircraft obtained their separation minimum with there still being enough time to escape.
This system was deemed more efficient because it did not try to predict
miss-distance, therefore the problem of accurate bearing measurement which
plagued the previous model was not a factor. Before reaching minimum separation and in
enough time to evade the intruder, an alarm would sound and tell the pilot to climb or
dive. The vertical component of the system operated with a small UHF transmitter which
periodically transmits a series of pulses. The pulses were spaced at different intervals
based on the aircrafts altitude. The receiver in the system interpreted the altitude
of any similarly equipped aircraft in the vicinity. Further analysis was required if an
aircraft was detected at or near the same altitude.5 During the development of
the Bendix system, Dr. John Smiley Morrell discovered and first used the
concept of Tau. Tau is based on time, not distance. Mathematically, Tau is
expressed as the range to the intruder divided by its closure rate or range-rate.6
The Bendix system only needed to determine the range of the aircraft at or near the same
altitude and the rate at which the range changed. Engineers did this by devising a system
called the ground-bounce ranging system. A transmitter sent a split signal one that
traveled directly to the receiver and another that bounced off of the ground, then to the
aircraft. The time delay between the direct signal and the ground-reflected signal was
calculated to determine how far apart the aircraft were. If the delay was short, the
aircraft were separated considerably, and if the delay was long, the aircraft were within
close proximity of each other. If the ratio of range to range-rate reached twice the
minimum escape time for the type aircraft on which the system was installed, the alarm
sounded and issue an instruction to climb or descend depending on whether the intruder was
higher or lower.7
Numerous other systems were considered for development. Eliminate Range-zero System (EROS)
was developed for fast moving, fighter-type aircraft. EROS used time-frequency techniques.
Each aircraft carried a very accurate and expensive cesium-rubidium clock that was
synchronized to a master clock. A pulse train of information (including the host
airplanes altitude) would be transmitted at a time precisely allocated for that
airplane. Based on the time differential measured when another airplane received the
signal, EROS could determine the range and closing speed of the approaching airplane. The
problem with this system, like all of the others that preceded it, was that it only
protected against aircraft with the same equipment on board. Since the system was so
costly, it was considered impractical and was never used.8
Since the mid 70s, efforts have concentrated on the use of hardware already
installed on most aircraft, namely the transponder of the Air Traffic Control Radar Beacon
System (ATCRBS). Basically, aircraft would be equipped with airborne interrogators that
would be able to interpret data from the transponders of nearby aircraft. These systems
became known as the Beacon Collision Avoidance System or BCAS. In the late 70s,
George Litchford, a New York electronics engineer, came up with a theory that a passive
anti-collision system could eavesdrop on ground interrogators and locate and track nearby
aircraft. It was given the name passive BCAS. This technique is based on listening
for transponder replies from other nearby aircraft to two or more ground interrogators. By
timing the receipt of these ground interrogations and replies from other aircraft, and
using the known positions of the ground interrogators, a passive system calculated the
relative positions and altitudes of other aircraft.9 Passive BCAS never
went into full production because it was considered too complex and would not work over
the ocean or where there was limited radar coverage. However, with the electronic and
navigational capabilities that exist today, there is hope for a passive TCAS system. This
is because in some instances, aircraft know exactly where they are if navigating with INS,
Loran-C, or GPS. In this case a passive system would only need to receive a signal from
one ground interrogator.

TRAFFIC ALERT/COLLISION AVOIDANCE SYSTEM

Building on this and other work, the FAA launched the TCAS program in 1981. TCAS is a
relatively simple system to understand. Basically, the system identifies the location and
tracks the progress of aircraft equipped with beacon transponders. Currently, there are
three versions of the TCAS system in use or in some stage of development; TCAS I, II, and
III. TCAS I, the simplest of the systems, is less expensive but also less capable than the
others. It was designed primarily for general aviation use. The TCAS I transmitter sends
signals and interrogates Mode-C transponders. The TCAS I receiver and display indicates
approximate bearing and relative altitude of all aircraft within the selected range,
usually about forty miles. Further, the system uses color coded dots to indicate which
aircraft in the area pose a potential threat. This is referred to as a Traffic Advisory
(TA). When a pilot receives a TA, it is up to him/her to visually identify the intruder
and is allowed to deviate up to + 300 feet. Lateral deviation is not authorized. In
instrument conditions, the pilot is required to notify air traffic control for assistance
in resolving the conflict.10 TCAS II on the other hand is a more comprehensive
system than TCAS I. This system was required to be installed on all commercial air
carriers operating in the United States by December 31, 1993. It offers all of the same
benefits but it will also issue a Resolution Advisory (RA) to the pilot. In other words,
the intruder target is plotted and the system is able to tell whether the aircraft if
climbing, diving, or in straight and level flight. Once this is determined, the system
will advise the pilot to execute an evasive maneuver that will deconflict the aircraft
from the intruder. There are two types of RAs, preventive and positive. Preventive RAs
instruct the pilot not to change altitude or heading to avoid a potential conflict.
Positive RAs instruct the pilot to climb or descend at a predetermined rate of 2500 feet
per minute to avoid a conflict.11 TCAS II is capable of interrogating Mode-C
and Mode-S. In the case of both aircraft having Mode-S interrogation capability, the TCAS
II systems communicate with one another and issue deconflicted RAs.12 Since
this system costs up to $200,000 per aircraft, manufacturers have built in an upgrade
capability to the next generation TCAS III. This system will be virtually the same as TCAS
II but will allow pilots who receive RAs to execute lateral deviations to evade intruders.
This will be possible because the directional antenna on TCAS III will be more accurate
and will have a smaller bearing error. There are also hopes that the new antenna will cut
down on false alarms since it can more accurately determine an intruders location.
Another upgrade that is proposed has to do with the Mode-S data link. Through this link, a
system will be capable of transmitting the aircrafts GPS position and velocity
vector to other TCAS-equipped aircraft thus providing much more accurate information.13

A FEW PROBLEMS AND SOLUTIONS

Needless to say, there were a few problems that occurred in the development of TCAS. There
was a problem with the directional capabilities of the antenna used with the system.
Signal clutter was also a big problem. Additionally, software upgrades had to be developed
to lessen the number of false alarms. Then lastly, but certainly not least, there were the
problems of getting pilots and controllers used to the system.
The antenna problem was a complex one. The typical spinning antennas that are located on
airports provide directional information to controllers. This data is available because
the antenna rotates 360 degrees at such a rate that the locations of aircraft can be
pinpointed every time the antenna makes a revolution. This philosophy is impractical for
airborne interrogators though. So, engineers developed an antenna that contains a
number of small antenna elements arranged in a circle around a center element. Fed
with the proper signal, they transmit an interrogating pulse simultaneously in all
directions. But when the responses arrive, they strike at slightly different times. By
comparing these patterns, of the returning signals at each element, the computer can find
the directions from whence the signals came.14
Signal clutter was another problem that had to be overcome. During early work on TCAS,
engineers were worried that in crowded terminal areas with many transponders replying to
multiple signals, the system would become overloaded with overlapping signals and clutter.
This problem was overcome with a process called the whisper-shout and with a
directional antenna. The whisper-shout method of interrogation allows the transmitter to
send signals in two strengths. A low power signal (the whisper) is transmitted and only
highly sensitive transponders, or transponders close by, can receive it and respond. Then
the transmitter sends a stronger signal (the shout) which triggers responses from less
sensitive transponders or those that are further away. The operative element in this
system is a mechanism that prohibits the transponders that responded to the whisper from
responding to the shout and vice-a-versa, thus reducing the number of transponders
responding at one time. A directional antenna was also incorporated into the system. This
antenna, described in the previous paragraph has the ability to transmit in only one
quadrant at a time thus reducing the number of signals being interrogated at any given
time. These two components were key elements in the development of TCAS and prevent system
overload even in the most crowded terminal areas.15
There was not much for support for TCAS II when it was first introduced because of the
large number of false conflict alerts. These were particularly disturbing because many
alerts occurred when aircraft were on final approach, one of the busiest and most critical
phases of flight. Version 6.00 was the original software for TCAS II. When using this
software, some very interesting problems occurred. False conflict alerts were being
triggered by transponders on ships and bridges. Additionally, parallel final approach
courses less than 5000 feet apart were causing false alerts. It has even been reported
that a pilots own aircraft can cause a false alarm. In this situation the pilot
found himself trying to outmaneuver himself. All of these are software problems and have
been addressed in the latest version, 6.04.16 Through Mitre Corporations
new logic-software version, Delta airlines, the first voluntary user, reported an 80
percent reduction in TCAS conflict alerts. Additionally, the number of Bump-up
alerts have been reduced. Bump-up alerts occur when the TCAS of a descending
aircraft calls for it to climb to avoid a fast-climbing aircraft below, not knowing that
the aircraft will level off at a lower altitude. This was a common occurrence at
Dallas-Fort Worth airport because arriving and departing aircraft use the same fixes.17
Additionally, the buffer requirements or thresholds between participating aircraft were
lowered, thus reducing the number of false conflict alerts.
We are all resistant to change. It is just a fact of life. This was especially the case
with TCAS. When TCAS was first introduced, it was viewed as a nuisance more than anything
else. This was because the users considered the system unreliable. Pilots viewed it as
just another instrument they had to watch in an already busy cockpit. They, in some cases,
became complacent and began to totally disregard TCAS conflict alerts which defeats the
whole purpose of the system. By reducing the number of unnecessary TCAS alerts, the new
software is expected to increase the confidence of flight crews in responding regularly to
TCAS alerts. Already, with the new software upgrade, pilots opinions are beginning to
sway. They have begun to consider TCAS as a way for them to increase their situational
awareness. It gives them the big picture on a screen in the cockpit; something they had to
develop mentally before.18 Additionally, it has been reported that TCAS has
been used to avoid wake turbulence by getting too close to heavy aircraft.

THE FUTURE OF TCAS

TCAS was developed to help reduce the potential for midair collisions. However, the time
could someday come when the system actually helps to relieve congestion and expedite
traffic as well. An example of this was tested on several occasions in 1993. The In-Trail
Climb (ITC) is intended to reduce fuel consumption and reduce separation criteria for
transoceanic flights. This maneuver permits a trailing aircraft at a lower altitude to
climb through the altitude of a preceding aircraft using TCAS II as a separation
maintenance aid. This is substantial because it allows aircraft to climb to more fuel
efficient or less turbulent cruising altitudes earlier in their flights. During the first
test last year, a United DC-10 was able to save 2000 lbs of fuel.19 Other
prospects for TCAS include reduced separation on transoceanic routes, reduced spacing for
departures in instrument conditions, and could permit aircraft to establish and maintain
separation intervals on final during approaches.20
Another intriguing prospect for the use of TCAS is that of being able transmit GPS
coordinates and altitude via Mode-S datalink. This information could be used to enhance
the effectiveness and accuracy of TCAS and could also be transmitted to air traffic
control by means other than conventional radar. The system would also be able to be
incorporated rapidly and at a minimum cost because only a software upgrade would be
required for those already using TCAS II. This again, would be especially useful for
transoceanic flights by relaying position information from aircraft to air traffic control
centers.21